Scientists
are currently uncertain as to whether homosexuality is primarily caused by
environmental or genetic factors. The uncertainty arises because much of the
available data involves correlations, human behavior is likely affected by
interactions (complexity) and humans are VERY difficult research subjects when
it comes to sex. But evidence is pointing to a combination of genetic and
environmental factors influencing sexual preference..

Some of the
most profound questions about humans address our behavior.From culture to crime, science to
society, we often want to know whether our destiny and who we are is in our
genes or a matter of choice and determined by our environment.Our parents went to great lengths to
teach us and equip us for this world, but many of those efforts would be wasted
if it was all in our genes.To
some extent, genes and environment are inseparable.But when it comes to why some of us are scientists and
others artists, or other differences in behavior, we can at least attempt to
partition genetic effects from environmental ones.As an analogy, if we want a hunting dog to retrieve birds,
there are some obvious breeds to choose and train, but many breeds will be
worthless for this task no matter how much they are trained.Behavior of humans is more challenging
to understand than that of other animals, but some of the same principles can
be applied to both humans and other animals.

The question of
whether a behavioral difference among people is due to genetic differences or
environmental differences is known as the question of biological determinism, often phrased as “is
it in our genes?” or as “nature vs. nurture.”Biological determinism is relevant to many issues of social
relevance:

crime

obesity

mental
illness

IQ

addiction

risk-taking

and some others
that will be given in class.There
are several reasons that answering the question of biological determinism is
intrinsically difficult.One is
that humans make difficult subjects, especially limiting the kinds of
experiments that can be done.This
limitation in turn means that we have to rely on correlations for much of our
conclusions..Finally, many of our
behaviors are due to a combination of many factors, with possible interactions
(complexity). Consequently, there has been little resolution of any of the
nature-nurture debate.

Your generation
is perhaps unaware of the dark side of biological determinism that was
manifested in the past.The most
notorious abuse was the eugenics program of Germany’s Nazi regime in the 1930s
and 1940s.The eugenics program
was used to justify genocide of the Jews as well as the killing of gypsies and
homosexuals and others deemed socially inferior.The U.S. was never that extreme, but throughout much of the
1900s, the eugenics movement led to sterilization (castration) of those deemed
mentally inferior.The motivation
for eugenics was to improve the gene pool, and castration was done to prevent
the inferior individuals from having babies.Yet, if the basis of their ‘inferior’ minds was
environmental – perhaps nothing more than a lack of education – then preventing
them from having offspring did nothing to improve the gene pool.

In this
chapter, the focus is on sexual preference.A person’s physical sex depends on gonad type (testes,
ovaries) as well as secondary sexual traits, such as genitalia (penis versus
clitoris and vagina), breast development, facial hair, and so forth..A person’s sexual preference is
measured by whether they prefer to have sex with someone of the same sex (=
homosexual preference, known as ‘gay’ if male and ‘lesbian’ if female), or
whether they prefer someone of the opposite sex (= heterosexual or straight). A
separate but related behavior concerns gender identity, which is whether a
person thinks of himself/herself as man or woman.Gender identity can be fully separated from sexual
preference.Individuals get sex
change operations because of issues with gender identity, but men who get
changed into women will sometimes have sexual preferences for women (for
example).

Being
gay/lesbian has social consequences, especially to the individuals with the
homosexual preferences.It is
estimated that 2%-5% of men are gay, 1%-2% of women are lesbian, and these
percentages appear to hold across cultures, as best one can tell.Despite the relative abundance of these
behaviors, many states have passed referenda that disallow same-sex
marriages.On a more individual
level, ‘gay-bashing’ has led to many deaths and less brutal beatings, based
entirely on widespread intolerance of homosexual preferences.Perhaps for all of these reasons, there
have been many searches for correlates of gay/lesbian behavior that might
provide some clues to what determines it.In the recent past, and thus likely still, it has been commonly thought
that homosexual preference is a choice and even learned: a 1970 U.S. survey
found that 43% thought that young gays learned their SP from older gays. Thus,
if we can find anatomical or physiological correlates of being gay, we may at
least settle the question of whether sexual preference is learned or a ‘choice.’

Overall, there
are a few patterns of association with homosexual preference, but they are weak
(hence demonstrated only statistically) and prone to poor repeatability.This topic is so far a difficult one to
research because of the lack of decisive patterns.Nonetheless, there is collective support for both
environmental and genetic causes.

Genetics

Several lines
of evidence suggest a weak-moderate genetic component to sexual
preference.A genetic basis is especially
difficult to establish for human behavioral differences, both because we don’t
do experimental crosses with people and because there is so much parental
influence on behavior that confounds environmental effects with genetic ones.One of the most useful comparisons
therefore makes use of identical twins versus non-identical (fraternal)
twins.Identical twins are
genetically the same, so any difference between a twin pair must be non-genetic
(environmental).Fraternal twins
are genetically related but not identical.Both kinds of twins share the womb and are the same age, so
they experience many environmental similarities that might be thought to affect
behavior.As a consequence, if
identical twins more often have the same type of behavior than fraternal twins,
we suspect a partial genetic basis to the behavior.If the behavior was 100% due to genes, two identical twins
should always have the same behavior.And if there is no genetic basis to the behavior, then identical twins
should no more often be similar to each other than fraternal twins.

The twin data
show that identical twins have about 50% concordance for sexual preference in
some studies, 30% in others. Fraternal twins have nearly half this
concordance.So these data suggest
that there is a modest effect of genetics.Other data, using a combination of molecular techniques and
pedigrees, suggest that an X-linked gene or region influences sexual
preference, but that finding has not been confirmed in all careful
studies.

Miscellaneous
Correlations

Fraternal
birth order.The probability
that a man has homosexual preference increases with the number of older
brothers he has.Each older
brother increases the odds by 1/3 – 1/2.This effect cannot have a genetic basis.Speculations for this effect focus on the mother
progressively building antibodies against an unknown male protein, more so with
each son.

Finger
length ratio.The ratio of the
index finger to the 4th finger is higher in women than men.In people with homosexual preference,
there is a tendency for the ratio to be lower than in heterosexuals of the same
sex.By this criterion,
homosexuality is associated with overmasculinization.

Childhood
gender non-conformity.Children that fail to conform to standard childhood gender roles (such
as ‘tomboy’ girls and effeminate boys) have a higher incidence of adult
homosexuality than children that conform to standard gender roles.This kind of study is difficult to do
properly (prospectively).The
danger of doing this kind of study retrospectively, after sexual preference is
already known, is that there will be a biased tendency to selectively recall
instances of childhood behaviors that fit the adult outcome.

Otoacoustic
emissions (OAE).Our ears
actually make sounds, though they are too weak to hear by ear.They have a characteristic frequency,
starting in early childhood.The
right ear’s OAE is different from the left ear’s, and males differ from
females.A UT researcher (Dennis
McFadden) is finding that gay males have slightly different OAE frequencies
than heterosexual men.The direction
of the difference supports an overmasculinization of gay men.

Neuroanatomy

There is a
strong temptation to think or hope that a behavioral difference as strong as
the difference between homosexual and heterosexual preferences will have a
physical manifestation in the brain.There is an increasing number of techniques that can be performed
non-invasively on a live person, but many of the most direct assays, and those
that can be applied to microscopic regions of the brain, require actual brain
material.There are obvious
difficulties in obtaining sufficient material for those studies, and work
repeating any findings work is rare.Studies have reported the following, but one should not consider any of
the patterns as demonstrated beyond reasonable doubt.

Interstitial
nucleus of the Anterior Hypothalamus #3.In 1991, Simon LeVay reported a search for sexual preference
differences in the size (volume) of 4 brain nuclei in a brain region known as
the anterior hypothalamus.Work on
rodents had demonstrated that this brain region affected sexual behavior, and
work on humans had already identified a male-female difference in tiny regions
or ‘nuclei’ of the anterior hypothalamus.LeVay found a difference between heterosexual and gay men in one of
these nuclei (#3); the size of INAH3 in gay men was similar to that of
(heterosexual) females and smaller than that of heterosexual men.

Other studies
have reported brain differences associated with sexual preference.The regions involved (in different
studies) have been the suprachiasmatic nucleus, the mid-saggital plane of the
anterior commissure, and the isthmus of the corpus callosum.

As mentioned
above, the biological basis of sexual preference in humans is a research area
in which it will be some time before we have definitive answers.The problems are

i)humans
are difficult subjects

a.they
do not readily divulge these sexual preferences

b.the
experiments done are limited

ii)because
of (i), we have only correlations to work with

iii)possible
complexity:there appear to be
many factors influencing sexual preference, none of which are strong (we don’t
know about interactions, but they may exist).